update NEWS for 0.4

[mlubin]

@blegat @odow @joaquimg let me know if there's a reason to hold off on this release. If we do wait, then we should stick to 0.3 for JuMP-dev.

[blegat]

No, I may suggest more changes before JuMP-dev (e.g. #397) but they are not breaking so it can be tagged as v0.4.1.
We should definitely tag v0.4 soon if we want solvers to work with v0.4 during the JuMP-dev meetup

[codecov-io]

Codecov Report

Merging #399 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   96.34%   96.36%   +0.01%     
==========================================
  Files          40       40              
  Lines        5066     5066              
==========================================
+ Hits         4881     4882       +1     
+ Misses        185      184       -1

Impacted Files	Coverage Δ
src/Utilities/functions.jl	97.9% <0%> (+0.52%)	⬆️

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